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A high rigidity glass-ceramic substrate is provided which contains, as a
predominant crystal phase or phases, at least one crystal phase selected
from the group consisting of enstatite (MgSiO), enstatite solid solution
(MgSiO.sub.3 solid solution), magnesium titanate (MgTi.sub.2O.sub.5) and
magnesium titanate solid solution (MgTi.sub.2O.sub.5 solid solution), has
fine crystal grains (preferably globular crystal grains) of precipitated
crystal phases, has excellent melting property of a base glass, high
resistivity to devitrification, easiness in polishing, excellent
smoothness in the surface after polishing and has high Young's modulus
capable of coping with a high speed rotation.

James V. Costigan, Esq.
HEDMAN & COSTIGAN, P.C.
Suite 2003
1185 Avenue of the Americas
New York
NY
10036-2646
US

Serial No.:

947960

Series Code:

09

Filed:

September 6, 2001

Current U.S. Class:

501/10; G9B/5.288

Class at Publication:

501/10

International Class:

C03C 010/02

Foreign Application Data

Date

Code

Application Number

Jun 1, 1999

JP

154052/1999

May 2, 2000

JP

133213/2000

Claims

What is claimed is:

1. A high rigidity glass-ceramic substrate having a predominant crystal
phase or phases, said predominant crystal phase being at least one
selected from the group consisting of enstatite (MgSiO,), enstatite solid
solution (MgSiO.sub.3 solid solution), magnesium titanate
(MgTi.sub.2O.sub.5) and magnesium titanate solid solution
(MgTi.sub.20.sub.5 solid solution), and glass-ceramic constituting the
glass-ceramic substrate having Young's modulus within a range from 115
GPa to 160 GPa and containing less than 20 weight percent of
M.sub.2O.sub.3 ingredient.

2. A high rigidity glass-ceramic substrate having a predominant crystal
phase or phases, said predominant crystal phase comprising enstatite
(MgSiO) or enstatite solid solution (MgSiO.sub.3 solid solution) as a
first crystal phase having the largest ratio of precipitation, and
glass-ceramic constituting the glass-ceramic substrate having Young's
modulus within a range from 115 GPa to 160 GPa and containing less than
20 weight percent of Al.sub.2O.sub.3 ingredient.

3. A high rigidity glass-ceramic substrate having a predominant crystal
phase or phases, said predominant crystal phase comprising magnesium
titanate (MgTi.sub.2O) or magnesium titanate solid solution
(MgTi.sub.2O.sub.5 solid solution) as a first crystal phase having the
largest ratio of precipitation, and glass-ceramic constituting the
glass-ceramic substrate having Young's modulus within a range from 115
GPa to 160 GPa and containing less than 20 weight percent of
Al.sub.2O.sub.3 ingredient.

4. A high rigidity glass-ceramic substrate having a predominant crystal
phase or phases, said predominant crystal phase comprising enstatite
(MgSiO.sub.3) or enstatite solid solution (MgSiO.sub.3 solid solution) as
a first crystal phase having the largest ratio of precipitation and at
least one selected from the group consisting of magnesium titanate
(MgTi.sub.2O), magnesium titanate solid solution (MgTi.sub.2O.sub.5 solid
solution), spinel and spinel solid solution as a second crystal phase
having a smaller ratio of precipitation than the first crystal phase, and
glass-ceramic constituting the glass-ceramic substrate having Young's
modulus within a range from 115 GPa to 160 GPa and containing less than
20 weight percent of Al.sub.2O.sub.3 ingredient.

5. A high rigidity glass-ceramic substrate having a predominant crystal
phase or phases, said predominant crystal phase comprising magnesium
titanate (MgTi.sub.2O.sub.5) or magnesium titanate solid solution
(MgTi.sub.2O) as a first crystal phase having the largest ratio of
precipitation and at least one selected from the group consisting of
enstatite (MgSiO) or enstatite solid solution (MgSiO.sub.3 solid
solution), spinel and spinel solid solution as a second crystal phase
having a smaller ratio of precipitation than the first crystal phase, and
glass-ceramic constituting the glass-ceramic substrate having Young's
modulus within a range from 115 GPa to 160 GPa and containing less than
20 weight percent of Al.sub.2O.sub.3 ingredient.

6. A high rigidity glass-ceramic substrate as defined in claim 1 wherein
said glass-ceramic is substantially free from Li.sub.2O, Na.sub.2O and
K.sub.2O.

7. A high rigidity glass-ceramic substrate as defined in claim 1 having a
surface roughness Ra (arithmetic mean roughness) after polishing of 8
.ANG. or below and a maximum surface roughness Rmax after polishing of
100 .ANG. or below.

8. A high rigidity glass-ceramic substrate as defined in claim 1 having a
coefficient of thermal expansion within a range from
40.times.10.sup.-7/.degree. C. to 60.times.10.sup.-7/.degree. C. within a
temperature range from -50.degree. C. to +70.degree. C.

9. A high rigidity glass-ceramic substrate as defined in claim 1 wherein
said predominant crystal phase has a crystal grain diameter within a
range from 0.05 .mu.m to 0.30 .mu.m.

10. A high rigidity glass-ceramic substrate as defined in claim 1 having
Vickers hardness within a range from 6860 N/mm.sup.2 to 8330 N/mm.sup.2.

11. A high rigidity glass-ceramic substrate as defined in claim 1 wherein
said glass-ceramic comprises in weight percent on the oxide basis:

12. A high rigidity glass-ceramic substrate as defined in claim 1 further
comprising an element selected from the group consisting of P, W, Nb, La,
Y and Pb in an amount up to 3 weight percent on the oxide basis and/or an
element selected from the group consisting of Cu, Co, Fe, Mn, Cr, Sn and
V in an amount up to 2 weight percent on the oxide basis.

13. A high rigidity glass-ceramic substrate as defined in claim 1 which is
provided by melting glass materials, forming and annealing a base glass
and subjecting the base glass to heat treatment for crystallization under
a nucleation temperature within a range from 650.degree. C. to
750.degree. C., a nucleation time within a range from one hour to twelve
hours, a crystallization temperature within a range from 850.degree. C.
to 1000.degree. C. and a crystallization time within a range from one
hour to twelve hours.

14. A magnetic information storage disk provided by forming a film of a
magnetic information storage medium on the high rigidity glass-ceramic
substrate as defined in claim 1.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to a high rigidity glass-ceramic substrate
and, more particularly, to a high rigidity glass-ceramic substrate for a
magnetic information storage medium used, e.g., for a magnetic
information storage device having a super flat substrate surface capable
of coping with near contact recording or contact recording employed in
the ramp loading system and capable also of coping with a high speed
rotation of a magnetic information storage medium. The invention relates
also to a magnetic information storage medium such as a magnetic disk
which is provided by forming a film of an information storage medium on
the glass-ceramic substrate.

[0002] In this specification, the term "magnetic information storage
medium" means a magnetic information storage medium in the form of a disk
and includes fixed type hard disks, removable type hard disks and card
type hard disks used respectively for so-called "hard disks" for personal
computers and also other disk type magnetic information storage media
which can be used in HDTV, digital video cameras, digital cameras, mobile
communication devices etc. In this specification, the term "spinel" means
at least one of (Mg and/or Zn)Al.sub.2O.sub.4, (Mg and/or
Zn).sub.2TiO.sub.4 and a mixture in the form of a solid solution between
these two crystals and the term "spinel solid solution" means a crystal
in which other ingredient is mixed with spinel and/or a part of spinel is
substituted by other ingredient.

[0003] Recent development of personal computers for multi-media purposes
and digital video cameras and digital cameras requires handling of a
large amount of data such as a moving picture and voice and, for this
purpose, a magnetic information storage device of a large recording
capacity is required. For increasing the recording density, there is a
tendency in the art of a magnetic information storage medium toward
reducing the size of a bit cell and thereby increasing the bit and track
density. As a result, the magnetic head performs its operation in closer
proximity to the disk surface. For coping with starting and stopping of a
magnetic head which is operated in a near contact state or a contact
state with respect to a magnetic information storage medium, a landing
zone system has been developed according to which a specific part (an
unrecorded part in a radially inside or outside portion of a disk) is
processed for preventing stiction of the magnetic head to the disk.

[0004] In the current magnetic information storage device, the CSS
(contact start stop) system is adopted according to which (1) the
magnetic head is in contact with the magnetic information storage medium
before starting its operation and, (2) when the magnetic head has started
its operation, the magnetic head flies over the surface of the magnetic
information storage medium. If the plane of contact between the magnetic
head and the magnetic information storage medium is exceedingly in the
state of a mirror surface, stiction of the magnetic head occurs resulting
in unsmooth starting of rotation due to increased friction and damage to
the surface of the magnetic information storage medium or the magnetic
head. Thus, the magnetic information storage medium faces conflicting
demands for lower flying height of the magnetic head accompanying
increase in the storage capacity and prevention of stiction of the
magnetic head to the surface of the magnetic information storage medium.
As an answer to satisfy such conflicting demands, a ramp loading
technique has been developed according to which the magnetic head in
operation is completely in contact with the surface of the magnetic
information storage medium but starting and stopping of the magnetic head
are performed in an area outside of the surface of the magnetic
information storage medium. Thus, there has been an increasing demand for
a smoother surface of the magnetic information storage medium.

[0005] Developments are also in progress for transferring information at a
higher speed by rotating a magnetic information storage medium at a
higher speed. Since, however, a high speed rotation of a substrate for
the magnetic information storage medium causes deflection and deformation
in the substrate, the substrate is required to have a higher Young's
modulus. Further, in addition to the currently used fixed type hard
disks, magnetic information storage devices which use removable type hard
disks and card type hard disks which require a high strength of the
substrate are being considered and becoming feasible and application of
the substrate to HDTV, digital video cameras, digital cameras and mobile
communication devices is under way.

[0006] In the situation in which a high rigidity substrate material is
required, an aluminum alloy substrate cannot provide a sufficient
strength and, if thickness of the substrate is increased, it will make it
difficult to realize a compact and light-weight design of the medium. For
solving the problem inherent in the aluminum alloy substrate, known in
the art are chemically tempered glasses such as alumino-silicate
(SiO.sub.2-Al.sub.2O.sub.3-Na.sub.2O) glass proposed by Japanese Patent
Application Laid-open Publication Nos. Hei 8-48537 and Hei 5-32431 etc.).
These chemically tempered glasses, however, have the following
disadvantages: (1) Since polishing is made after the chemical tempering
process, the chemically tempered layer is seriously instable in making
the disk thinner. (2) Since the chemically tempered phase causes aging
when used for a long time, magnetic properties of the magnetic
information storage medium are deteriorated. (3) Since the glass contains
Na.sub.2O or K.sub.2O ingredient as an essential ingredient, these alkali
ingredients diffuse in the film formed during the film forming process
and thereby deteriorate magnetic properties of the magnetic information
storage medium. For preventing this, a barrier coating over the entire
surface for preventing diffusion of Na.sub.2O or K.sub.2O becomes
necessary and this makes it difficult to produce the product in a stable
manner at a low cost. (4) Chemical tempering is made for improving
mechanical strength of the glass but this is based on utilization of
tempering stress between the surface phase and the inside phase. Young's
modulus of the chemically tempered glass is 830 GPa or below which is
equivalent to ordinary amorphous glass and this poses limitation to
application of the chemically tempered glass to a high speed rotation
drive. Thus, the chemically tempered glasses are not sufficient as a
substrate for a high recording density magnetic information storage
medium.

[0007] Aside from the aluminum alloy substrates and chemically tempered
substrates, known in the art are some glass-ceramic substrates. For
example, glass-ceramic substrates disclosed in Japanese Patent
Application Laid-open Publication No. Hei 9-35234 and EP0781731A1 have a
Li.sub.2O--SiO.sub.2 composition and contain lithium disilicate and
.beta.-spodumene crystal phases or lithium disilicate and
.beta.-cristobalite crystal phases. In the glass-ceramic substrates,
however, no consideration or suggestion is made about relation between
Young's modulus and specific gravity al all. Young's modulus of these
glass-ceramics is 100 GPa at the maximum.

[0008] For improving such low Young's modulus, Japanese Patent Application
Laid-open Publication No. Hei 9-77531 discloses a glass-ceramic of a
SiO.sub.2--Al.sub.2O.sub.3-MgO-ZnO-TiO.sub.2 system and a rigid disk
substrate for a magnetic information storage medium. This glass-ceramic
contains a large quantity of spinel as a predominant crystal phase and
also contains MgTi.sub.2O.sub.5 and other crystal phases as sub-crystal
phases and has Young's modulus of 96.5-165.5 GPa. In this material, the
predominant crystal phase is only spinel represented by
(Mg/Zn)Al.sub.2O.sub.4 and/or (Mg/Zn),TiO.sub.4 and the sub-crystal
phases are not limited to specific crystals but crystals of a broad range
are mentioned. Further, this glass-ceramic contains a large amount of
Al.sub.2O.sub.8 and is different from the glass-ceramics of the present
invention which contain a relatively small amount of Al.sub.2O.sub.3 and
have a high Young's modulus and resistivity to devitrification. Such
large amount of Al.sub.2O.sub.3 deteriorates melting property of a base
glass and resistivity to devitrification and thereby adversely affects
productivity. The proposed glass-ceramic, therefore, is a merely rigid
material. Furthermore, the glass-ceramic of this system has an
exceedingly high surface hardness (Vickers hardness) and this adversely
affects processability and large scale production. Accordingly, the
improvement achieved by this substrate material is still insufficient for
a substrate of a high recording density magnetic information storage
medium.

[0009] WO98/22405 publication discloses a glass-ceramic of a Si
.sub.2-Al.sub.2O.sub.3-MgO-XZrO.sub.2-TiO.sub.2-Li.sub.2O system. This
glass-ceramic contains .beta.-quarts solid solution as a predominant
crystal phase and enstatite, spinel and other crystals as sub-crystal
phases and has a crystal grain diameter of 1000 .ANG. or below. This
glass-ceramic, however, requires Li.sub.2O as an essential ingredient in
its composition and, besides, requires .beta.-quarts solid solution as
its predominant crystal phase and, therefore, is entirely different from
the glass-ceramics of the present invention.

[0010] It is, therefore, an object of the present invention to overcome
the problems of the prior art substrates and provide a high rigidity
glass-ceramic substrate which is suitable for a substrate of a magnetic
information storage medium having excellent surface characteristics
capable of coping with the ramp loading system (i.e., contact recording
of the magnetic head) for high density recording and having a high
Young's modulus characteristics capable of coping with a high speed
rotation and surface hardness characteristics suitable for processing.

[0011] It is another object of the invention to provide a magnetic
information storage disk provided by forming a magnetic information
storage film on such glass-ceramic substrate.

SUMMARY OF THE INVENTION

[0012] Accumulated studies and experiments made by the inventors of the
present invention for achieving the above described objects of the
invention have resulted in the finding, which has led to the present
invention, that a high rigidity glass-ceramic substrate can be provided
which contains, as a predominant crystal phase or phases, at least one
crystal phase selected from the group consisting of enstatite
(MgSiO.sub.3), enstatite solid solution (MgSiO, solid solution),
magnesium titanate (MgTi.sub.2O) and magnesium titanate solid solution
(MgTi.sub.2O.sub.5 solid solution), has fine crystal grains (preferably
globular crystal grains) of precipitated crystal phases, has excellent
melting property of a base glass, high resistivity to devitrification,
easiness in polishing, excellent smoothness in the surface after
polishing and has high Young's modulus capable of coping with a high
speed rotation.

[0013] For achieving the objects of the invention, there is provided a
high rigidity glass-ceramic substrate having a predominant crystal phase
or phases, said predominant crystal phase being at least one selected
from the group consisting of enstatite (MgSiO3), enstatite solid solution
(MgSiO, solid solution), magnesium titanate (MgTi.sub.2O.sub.5) and
magnesium titanate solid solution (MgTi.sub.2O.sub.5 solid solution), and
glass-ceramic constituting the glass-ceramic substrate having Young's
modulus within a range from 115 GPa to 160 GPa and containing less than
20 weight percent of Al.sub.2O.sub.3 ingredient.

[0014] In one aspect of the invention, there is provided a high rigidity
glass-ceramic substrate having a predominant crystal phase or phases,
said predominant crystal phase comprising enstatite (MgSiO.sub.3) or
enstatite solid solution (MgSiO.sub.3 solid solution) as a first crystal
phase having the largest ratio of precipitation, and glass-ceramic
constituting the glass-ceramic substrate having Young's modulus within a
range from 115 GPa to 160 GPa and containing less than 20 weight percent
of Al.sub.2O.sub.3 ingredient In another aspect of the invention, there
is provided a high rigidity glass-ceramic substrate having a predominant
crystal phase or phases, said predominant crystal phase comprising
magnesium titanate (MgTi.sub.2O5) or magnesium titanate solid solution
(MgTi.sub.2O.sub.5 solid solution) as a first crystal phase having the
largest ratio of precipitation, and glass-ceramic constituting the
glass-ceramic substrate having Young's modulus within a range from 115
GPa to 160 GPa and containing less than 20 weight percent of
Al.sub.2O.sub.3 ingredient.

[0015] In another aspect of the invention, there is provided a high
rigidity glass-ceramic substrate having a predominant crystal phase or
phases, said predominant crystal phase comprising enstatite (MgSiO.sub.3)
or enstatite solid solution (MgSiO.sub.3 solid solution) as a first
crystal phase having the largest ratio of precipitation and at least one
selected from the group consisting of magnesium titanate
(MgTi.sub.2O.sub.5), magnesium titanate solid solution (MgTi.sub.2O.sub.5
solid solution), spinel and spinel solid solution as a second crystal
phase having a smaller ratio of precipitation than the first crystal
phase, and glass- ceramic constituting the glass-ceramic substrate having
Young's modulus within a range from 115 GPa to 160 GPa and containing
less than 20 weight percent of Al.sub.2O.sub.3 ingredient.

[0016] In another aspect of the invention, there is provided a high
rigidity glass-ceramic substrate having a predominant crystal phase or
phases, said predominant crystal phase comprising magnesium titanate
(MgTi.sub.2O.sub.5) or magnesium titanate solid solution (MgTi.sub.2O) as
a first crystal phase having the largest ratio of precipitation and at
least one selected from the group consisting of enstatite (MgSiO.sub.3)
or enstatite solid solution (MgSiO.sub.3 solid solution), spinel and
spinel solid solution as a second crystal phase having a smaller ratio of
precipitation than the first crystal phase, and glass-ceramic
constituting the glass-ceramic substrate having Young's modulus within a
range from 115 GPa to 160 GPa and containing less than 20 weight percent
of Al.sub.2O.sub.3 ingredient.

[0017] In another aspect of the invention, said glass-ceramic is
substantially free from Li.sub.2O, Na.sub.2O and K.sub.2O.

[0018] In another aspect of the invention, the high rigidity glass-ceramic
substrate has a surface roughness Ra (arithmetic mean roughness) after
polishing of 8 .ANG. or below and a maximum surface roughness Rmax after
polishing of 10 .ANG. or below.

[0019] In another aspect of the invention, the high rigidity glass-ceramic
substrate has a coefficient of thermal expansion within a range from
40.times.10.sup.-7/.degree. C. to 60.times.10.sup.-7/.degree. C. within a
temperature range from -50.degree. C. to +70.degree. C.

[0020] In another aspect of the invention, said predominant crystal phase
has a crystal grain diameter within a range from 0.059 .mu.m to 0.30
.mu.m.

[0021] In another aspect of the invention, the high rigidity glass-ceramic
substrate has Vickers hardness within a range from 6860 N/mm.sup.2 to
8330 N/mm.sup.2.

[0022] In another aspect of the invention, the glass-ceramic comprises in
weight percent on the oxide basis:

[0023] In another aspect of the invention, the high rigidity glass-ceramic
substrate further comprises an element selected from the group consisting
of P, W, Nb, La, Y and Pb in an amount up to 3 weight percent on the
oxide basis and/or an element selected from the group consisting of Cu,
Co, Fe, Mn, Cr, Sn and V in an amount up to 2 weight percent on the oxide
basis.

[0024] In another aspect of the invention, the high rigidity glass-ceramic
substrate is provided by melting glass materials, forming and annealing a
base glass and subjecting the base glass to heat treatment for
crystallization under a nucleation temperature within a range from
650.degree. C. to 750.degree. C., a nucleation time within a range from
one hour to twelve hours, a crystallization temperature within a range
from 850.degree. C. to 1000.degree. C. and a crystallization time within
a range from one hour to twelve hours.

[0025] In still another aspect of the invention, there is provided a
magnetic information storage disk provided by forming a film of a
magnetic information storage medium on the above described high rigidity
glass-ceramic substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Reasons for limiting physical properties, predominant crystal
phases, crystal grain diameter, surface characteristics and composition
of the glass-ceramics of the present invention will now be described. The
composition of the glass-ceramic is expressed on the basis of composition
of oxides.

[0027] Description will be made first about Young's modulus. As described
previously, for improving recording density and data transfer speed,
there is tendency toward a higher speed rotation of a magnetic
information storage medium. For coping with such tendency, the substrate
must have high rigidity and low specific gravity for preventing vibration
of a disk caused by deflection of the disk during a high speed rotation.
If the substrate has high rigidity but large specific gravity, deflection
of the disk occurs during a high speed rotation due to its large weight
which will result in vibration. Conversely, if the disk has small
specific gravity but low rigidity, vibration will similarly take place.

[0028] In the glass-ceramic substrate of the present invention having the
above described predominant crystal phase or phases, there is the
tendency that, if ingredients are adjusted to increase rigidity highly,
specific gravity will increase to an excessive degree whereas if
ingredients are adjusted to decrease specific gravity largely, rigidity
will decrease to an undesirable degree. Accordingly, a balance must be
found between rigidity and specific gravity so that apparently
conflicting requirements for high rigidity and low specific gravity will
both be satisfied. As a result of studies and experiments, it has been
found that Young's modulus should be 115 GPa or over while low specific
gravity is maintained and Young's modulus should not exceed 160 GPa
having regard to the balance with specific gravity. More preferably,
Young's modulus should be 120 GPa or over and 150 GPa or below. A
preferable value of Young's modulus/specific gravity is 37 GPa or over
and a more preferable value thereof is 39 GPa or over. The substrate
should preferably have specific gravity of 3.3 or below and, more
preferably, 3.1 or below.

[0029] A higher Young's modulus generally causes an increased surface
hardness of the material. An excessive surface hardness results in
prolonged processing time in polishing the material and this will
adversely affect productivity and prevent a low cost production. Having
regard to the influence of processability on productivity, the surface
hardness (`Vickers hardness) of the substrate should preferably be within
a range from 6860 N/mm.sup.2 to 8330 N/mm.sup.2.

[0030] As to Li.sub.2O, Na.sub.2O and K.sub.2O, if these ingredients are
included in a magnetized film (particularly perpendicular magnetized
film) which is required to have higher precision and finer quality, ions
of these ingredients will diffuse in the film during the film forming
process which will cause abnormal growth of magnetized film grains or
deterioration in the orientation. It is therefore important in the
substrate of the present invention to be substantially free from these
ingredients.

[0031] Description will now be made about predominant crystal phases. It
is a feature of the invention that the substrate contains, as its
predominant crystal phase or phases, at least one crystal phase selected
from the group consisting of enstatite (MgSiO), enstatite solid solution
(MgSiO.sub.3 solid solution), magnesium titanate (MgTi.sub.2O.sub.5) and
magnesium titanate solid solution (MgTi.sub.2O.sub.5 solid solution).
This is because these crystal phases have the advantages that they
contribute to increasing rigidity of the material and precipitating
crystal grains of a small grain diameter and also have sufficient
processability in the polishing process.

[0032] For achieving the above described physical properties, it is
preferable for the substrate to contain enstatite (MgSiO.sub.3) or
enstatite solid solution (MgSiO.sub.3 solid solution), magnesium titanate
(MgTi.sub.2O) or magnesium titanate solid solution (MgTi.sub.2O.sub.5
solid solution) as the first crystal phase having the largest ratio of
precipitation.

[0033] In a case where the first crystal phase is enstatite or enstatite
solid solution, the second crystal phase having a smaller ratio of
precipitation than the first crystal phase should preferably be at least
one selected from the group consisting of magnesium titanate, magnesium
titanate solid solution, spinel and spinel solid solution. In a case
where the first crystal phase is magnesium titanate or magnesium titanate
solid solution, the second crystal phase should preferably be at least
one selected from the group consisting of enstatite, enstatite solid
solution, spinel and spinel solid solution.

[0034] Spinel includes MgAl.sub.2O.sub.4, MgTiO.sub.4 or a mixture in the
form of a solid solution of these materials and spinel solid solution
includes a crystal in which other ingredient is mixed with spinel or a
crystal in which a part of spinel is substituted by other ingredient.

[0035] For achieving desirable physical properties and processability, a
particularly preferably form of the substrate in terms of the predominant
crystal phase is one containing magnesium titanate or magnesium titanate
solid solution as the first crystal phase having the largest ratio of
precipitation and enstatite or enstatite solid solution as the second
crystal phase having a smaller ratio of precipitation and containing no
spinel or spinel solid solution. The glass-ceramic having this form of
crystal phases can be obtained by adopting a composition containing 2-5%
BaO and 0.5-5% ZrO.sub.2.

[0036] Description will now be made about the crystal grain diameter of
the precipitated crystal phases and the surface roughness. As described
previously, for coping with the near contact recording system or the
contact recording system for improving recording density, the magnetic
information storage medium must have a more flat surface than the prior
art medium. If one attempts to perform high recording density inputting
and outputting of information on a magnetic information storage medium
having a surface of the prior art flatness, a high recording density
magnetic recording cannot be achieved because distance between the
magnetic head and the medium is too large. If this distance is reduced,
collision of the magnetic head against the surface of the medium occurs
with resulting damage to the head and medium. For this reason, for
achieving the flatness of the substrate surface capable of coping with
the near contact recording system or the contact recording system, it has
been found that the surface roughness Ra should be 8 .ANG. or below and
the maximum surface roughness Rmax should be 100 .ANG. or below. More
preferably, the surface roughness Ra should be 4 .ANG. or below and the
maximum surface roughness Rmax should be 50 .ANG. or below and most
preferably, the surface roughness Ra should be 2.5 .ANG. or below and the
maximum surface roughness Rmax should be 35 .ANG. or below.

[0037] Description will now be made about the coefficient of thermal
expansion. In increasing the bit and track densities and thereby reducing
the size of the bit cell, difference in the coefficient of thermal
expansion between the medium and the substrate has a great influence.
This physical property is greatly influenced by the type of crystal
phases grown and the ratio and amount of precipitation of the crystal
phases. Having regard to the coefficient of thermal expansion of the
medium and the crystal phases of the glass-ceramic substrate of the
present invention, a proper coefficient of thermal expansion in the
temperature range from -50.degree. C. to +70.degree. C. is within a range
from 40.times.10.sup.-7/.degree. C. to 60.times.10.sup.-7/.degree. C.

[0038] Reasons for limiting the composition range will now be described.

[0039] The SiO.sub.2 ingredient is a very important ingredient for growing
enstatite or enstatite solid solution as a predominant crystal phase by
heat treating the base glass. If the amount of this ingredient is below
40%, the grown crystal phase of the glass-ceramic obtained is instable
and its texture tends to become coarse and, further, resistivity to
devitrification of the base glass is deteriorated. If the amount of this
ingredient exceeds 60%, difficulty arises in melting and forming of the
base glass.

[0040] The MgO ingredient is a very important ingredient for growing
enstatite, enstatite solid solution, magnesium titanate or magnesium
titanate solid solution as a predominant crystal phase and also growing
spinel or spinel solid solution by heat treating the base glass. If the
amount of this ingredient is below 10%, a desired crystal cannot be
obtained and, if obtained, the grown crystal of the glass-ceramic is
instable and its texture tends to become coarse and, further, its melting
property is deteriorated. If the amount of this ingredient exceeds 20%,
resistivity to devitrification is deteriorated.

[0041] The Al.sub.2O.sub.3 ingredient is a very important ingredient for
growing enstatite solid solution or magnesium titanate sollid solution as
a predominant crystal phase and also growing spinel or spinel solid
solution by heat treating the base glass. If the amount of this
ingredient is below 10%, a desired crystal cannot be obtained and, if
obtained, the grown crystal of the glass-ceramic is instable and its
texture tends to become coarse and, further, its melting property is
deteriorated. If the amount of this ingredient is 20% or over, melting
property and resistivity to devitrification of the base glass are
deteriorated and, moreover, spinel becomes predominant as the first
crystal phase resulting in significant increase in the hardness of the
substrate which is undesirable for processability. A preferable range of
this ingredient is 10% to less than 18% and a more preferable range
thereof is 10% to 17%.

[0042] The CaO ingredient is an ingredient which improves the melting
property of the glass and prevents the grown crystal from becoming
coarse. If the amount of this ingredient is below 0.5%, such effects
cannot be obtained whereas if the amount of this ingredient exceeds 4%,
the grown crystal tends to become coarse, the crystal phase tends to
change and chemical durability is deteriorated.

[0043] The SrO ingredient is added for improving the melting property of
the glass. If the amount of this ingredient is below 0.5%, this effect
cannot be obtained. Addition of this ingredient up to 4% will suffice.
The BaO ingredient is preferably added for improving the melting property
of the glass. The amount of this ingredient is preferably 0.5% or over
and more preferably 2% or over. Addition of this ingredient up to 5% will
suffice.

[0044] The ZrO.sub.2 and TiO2 ingredients are important ingredients which,
in addition to a function as a nucleating agent, are effective for making
the grown crystals fine, improving the mechanical strength and improving
chemical durability. The ZrO.sub.2 ingredient should preferably be added
in the amount of 0.5% or over and addition of up to 5% will suffice. As
to the TiO2 ingredient, if the amount of this ingredient is 8% or below,
softening tends to occur during the crystallization process and, if the
amount of this ingredient exceeds 12%, melting of the base glass becomes
difficult and resistivity to devitrification is deteriorated.

[0045] The Bi.sub.2O.sub.3 ingredient is effective for preventing
devitrification of the base glass without impairing the melting property
and formability of the base glass. If the amount of this ingredient
exceeds 6%, erosion of Pt or SiO.sub.2 of the melting pot becomes
significant.

[0046] The Sb.sub.2O.sub.3 and As.sub.2O.sub.3 ingredients may be used as
refining agents in melting of the glass. Addition of each of these
ingredients up to 1% will suffice.

[0047] For adjusting properties of the glass-ceramics and for other
purposes, an element selected from the group consisting of P, W, Nb, La,
Y and Pb may be added up to 3% on the oxide basis and an element selected
from the group consisting of Cu, Co, Fe, Mn, Cr, Sn and V may be added up
to 2% on the oxide basis respectively within a range in which the
properties of the glass-ceramics will not be impaired.

EXAMPLES

[0048] Examples of the present invention will be described below.

[0049] Tables 1 to 4 show examples (No. 1 to No. 9) of compositions of the
high rigidity glass-ceramic substrate made according to the invention and
also comparative examples of the prior art compositions (Comparative
Example No. 1 for the alumino-silicate glass (chemically tempered glass)
disclosed by Japanese Patent Application Laid-open Publication No. Hei
8-48537, Comparative Example No. 2 for the Li.sub.2O-SiO.sub.2
glass-ceramics disclosed by Japanese Patent Application Laid-open No. Hei
9-35234 and Comparative Example No. 3 for the SiO.sub.2-Al.sub.2O.sub.3-M-
gO-ZnO-TiO.sub.2 glass-ceramics disclosed by Japanese Patent Application
Laid-open Publication No. Hei 9-77531 together with nucleation
temperature, crystallization temperature, crystal phases, crystal grain
diameter, Young's modulus, Vickers hardness, specific gravity, surface
roughness Ra after polishing, maximum surface roughness Rmax after
polishing and coefficient of thermal expansion within the temperature
range from -50.degree. C. to +70.degree. C. The ratio of precipitation of
the respective crystal phases were obtained by preparing 100% crystal
reference specimens of each crystal phase and measuring diffraction peak
areas by an X-ray diffractometer (XRD) using the internal reference
method. The crystal grain diameter was obtained by a transmission
electron microscope (TEM). The crystal type of crystal grains was
determined by the TEM structure analysis. The surface roughness Ra was
determined by an atomic force microscope (AFM). In the tables, the
crystal phases were described in the order of magnitude of the ratio of
precipitation. The order of the ratio of precipitation was determined by
the height of main peak of the respective crystal phases obtained by
X-ray diffraction. In Example Nos. 1 to 4, magnesium titanate solid
solution is abbreviated as "magnesium titanate SS", spinel solid solution
as "spinel SS" and, as to solid solutions of other crystals, solid
solution portions after the names of crystals are abbreviated as "SS"
(e.g., .beta.-quartz solid solution is abbreviated as ".beta.-quartz
SS").

[0053] For manufacturing the glass-ceramic substrate of the above
described examples, materials including oxides, carbonates and nitrates
are mixed and melted in conventional melting apparatus at a temperature
within a range from about 1350.degree. C. to about 1490.degree. C. The
molten glass is stirred to homogenize it and thereafter formed into a
disk shape and annelaed to provide a formed glass. Then, the formed glass
is subjected to heat treatment to produce the crystal nucleus under a
temperature within the range from 650.degree. C. to 750.degree. C. for
about one to twelve hours and then is further subjected to heat treatment
for crystallization under a temperature within the range from 850.degree.
C. to 1000.degree. C. for about one to twelve hours to obtain a desired
glass-ceramic. Then, the glass-ceramic is lapped with lapping grains
having an average grain diameter of 5-30 .mu.m for about 10 minutes to 60
minutes and then is finally polished with a cerium oxide grains or
zirconia grains having an average grain diameter of 0.5-2 .mu.m for about
30 minutes to 60 minutes.

[0054] As shown in Tables 1 to 4, the glass-ceramics of the present
invention are different from the prior art comparative examples of
alumino-silicate chemically tempered glass, Li.sub.2O-SiO.sub.2
glass-ceramics and SiO.sub.2-Al.sub.2O.sub.3-MgO-ZnO-TiO.sub.2
glass-ceramics in the crystal phase of the glass-ceramics. In comparison
of Young's modulus, the glass-ceramics of the present invention have
higher rigidity than the alumino-silicate chemically tempered glass and
the Li.sub.2O-SiO.sub.2 glass-ceramics. The SiO.sub.2-Al.sub.2O.sub.3-MgO-
-ZnO-TiO.sub.2 glass-ceramics of the Comparative Example No. 3 has such a
high surface hardness (Vickers hardness of 9800 N/mm.sup.2) that a
desired surface roughness cannot be obtained by the normal polishing
method. In contrast, the glass-ceramics of the present invention have
Vickers hardness of 8330 N/mm.sup.2 or below and can be polished to a
sufficiently smooth surface by the normal polishing method and, moreover,
have no defects such as anisotropy, foreign matters and impurities and
have a dense, uniform and fine texture (having a crystal grain diameter
of 0.3 .mu.m or below) and sufficient chemical durability to rinsing by
various chemicals and water or etching.